Patch use and vigilance behaviour by Nubian ibex: The role of the effectiveness of vigilance

Background: Brown (1999) presented a model for optimal levels of vigilance in foragers exploiting depletable resource patches. This theory includes vigilance along with time allocation, patch use, resource depletion, and energetic costs in a fitness-maximizing model. It predicts a humped-shaped relationship between the optimal level of vigilance and the effectiveness of vigilance. Aim: To test Brown's model with free-ranging Nubian ibex (Capra nubiana). Use specific predictions for ibex, including vigilance levels increasing and patch use decreasing with predation risk and with energetic state of the forager. Site of experiments: Negev Desert, Israel. Methods: Manipulate sight lines (lines between food and safety) to reduce the effectiveness of vigilance of Nubian ibex. Evaluate the resulting effects on foraging costs, patch use, and risk management behaviour (vigilance and apprehension). Quantify ibex vigilance by direct observations. Quantify patch use and apprehension by measuring giving-up densities (GUD, the amount of food left in a patch following exploitation) in artificial food patches that differ in complexity. We couple our manipulation of sight lines (effectiveness of vigilance) with placement of feeding trays near and far from a refuge (to manipulate the risk of predation) and with augmentation of food (to manipulate the energetic state of the forager). Results: Brown's model was supported. Nubian ibex were more vigilant and/or more apprehensive, and depleted food patches less thoroughly (left higher GUDs) in patches farther from a cliff (risky), in the presence of sight-line obstructions (low effectiveness of vigilance), and when food was augmented (low marginal value of energy).